Stem cells regenerate tissue by dividing asymmetrically, producing more stem cells (self- renewal) as well as differentiating daughters. Although differentiation is usually considered irreversible, there is increasing evidence that the rules of irreversibility can be broken following injury or in cell culture. The conversion of a differentiated cell to a less differentiated cell type, or dedifferentiation, endows certain organisms with remarkable regenerative properties. Similarly, cells of one type can often be induced to adopt a new fate even that of another lineage, in a process called transdifferentiation. Despite centuries of investigation, however, dedifferentiation and transdifferentiation are poorly understood at the molecular level, particularly in situations where they occur in intact tissues. We use Drosophila spermatogenesis as a model stem cell system, since it parallels mammalian spermatogenesis, yet we can precisely locate the sperm-producing spermatogonial (or germ line stem cells, GSCs) and manipulate their microenvironment (niche) genetically. In this niche, local cytokine and TGF-beta signaling promotes stem cell renewal, while cells leaving the niche differentiate. By manipulating GSC maintenance genetically in vivo we have discovered a surprising degree of plasticity in this lineage; differentiating spermatogonia can reverse their path and dedifferentiate into GSCs. Additional forms of damage also trigger novel mechanisms of stem cell regeneration in this tissue. Since cell fate transformation (or plasticity) may be a general feature of many stem cell systems, but is often difficult to study in intact tissues, we propose to use the powerful tools of Drosophila genetics to study these events. This work will begin to reveal the molecular mechanisms by which differentiating cells can be coaxed to reverse their path and become functional stem cells. This will advance the field of regenerative medicine and also further our understanding of spermatogonial stem cell renewal, a fundamental aspect of male reproduction.